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Q41. Swell waves with a period of 15 seconds arrive on a beach. On the next day at the same time the period of the waves decreased to 10 seconds. How far away (km) was the storm that generated these waves? (Given: Wave speed C = 1.5 T)
Swell travels at group velocity Cg = C/2 = 0.75 * T. Cg1 = 11.25 m/s. Cg2 = 7.5 m/s. Let Distance = D and Time = t. D = 11.25 * t and D = 7.5 * (t + 86400). Solving these equations yields D = ~1,944,000 m, which is approximately 2000 km.
Q42. Which of the following represents the correct order of ocean currents in the surface layer of oceanic conveyor belt?
Surface return flow travels from the Pacific via the Indonesian throughflow, enters the Indian Ocean, sweeps around South Africa via the Agulhas current, flows up Africa's coast (Benguela), and eventually feeds the Gulf Stream.
Q43. From thermodynamic criteria of free energy change, calcite is predicted to precipitate in seawater. What is the free energy requirement for the DISSOLUTION to happen given deltaG_Ca2+ = -553.54, deltaG_CO3 = -527.9, deltaG_CaCO3 = -1128.94 (all kJ/mol)?
Standard free energy of dissolution = (-553.54 + -527.9) - (-1128.94) = +47.5 kJ/mol. Using activities (Q = 10^-8) at 27C (300K), RTlnQ = -45.9 kJ/mol. Total Delta G = 47.5 - 45.9 = +1.6 kJ/mol. Because Delta G is positive, it REQUIRES about 1.5 kJ/mol to force dissolution.
Q44. Compared to an electromagnetic wave of frequency 1.2 MHz travelling in a medium of resistivity 100 ohm-m, the skin depth of a wave of 2.4 MHz travelling in a medium of resistivity 50 ohm-m is:
Skin depth is proportional to sqrt(Resistivity / Frequency). Medium 1: sqrt(100/1.2). Medium 2: sqrt(50/2.4). Ratio = sqrt((50/2.4)/(100/1.2)) = sqrt(1/4) = 1/2. It decreases by a factor of 2.
Q45. An earthquake occurring at every 100 years, leads to a displacement of 2 m over a 40 km long fault. The corresponding shear strain accumulation rate is:
Shear strain = Displacement / Fault Length = 2m / 40,000m = 5 x 10^-5. Since this strain accumulates over 100 years, the annual rate is (5 x 10^-5) / 100 = 0.5 x 10^-6 per year.
Q46. Sedimentary exhalative (SEDEX) Pb-Zn-Ag ore deposits are associated with:
SEDEX deposits typically form in intra-continental or epicratonic rift basins where hot, metalliferous brines are exhaled onto the seafloor, precipitating layered sulfides.
Q47. What are the appropriate names respectively, for fine-grained rocks consisting of: (a) 29% sodic plagioclase, 40% alkali feldspar, 15% nepheline, 12% aegirine-augite and 4% opaque (b) 45% plagioclase (An25), 8% alkali feldspar, 30% quartz, 5% hornblende, 8% biotite and 4% opaque
Rock (a) has abundant alkali feldspar and nepheline, classifying it as Phonolite. Rock (b) is quartz-rich with plagioclase > alkali feldspar, classifying it as Dacite.
Q48. Which one of the following order of residence times of the elements in the seawater is correct?
Residence times depend on reactivity and biological use. Silica (Si) is rapidly consumed by diatoms (~20k yrs). Calcium (Ca) is used for shells (~1M yrs). Magnesium (Mg) is highly soluble (~13M yrs). Chlorine (Cl) is essentially conservative (>100M yrs).
Q49. In which of the following conditions relative sea level rises?
Relative sea level is the difference between global (eustatic) sea level and local tectonic elevation. If the basin floor drops (subsides) faster than the ocean water level falls, the net local effect is a relative rise in sea level.
Q50. In a two layered Earth, if the acoustic impedance values of layers are interchanged, then:
Reflection coefficient R = (Z2 - Z1)/(Z2 + Z1). Swapping Z1 and Z2 flips the sign. Transmission T = 2*Z1/(Z2 + Z1). Since acoustic impedance is always positive, T is always positive and does not change sign.